Biplane ultrasonic imaging

ABSTRACT

An ultrasonic apparatus and method are described in which a volumetric region of the body is imaged by biplane images. One biplane image has a fixed planar orientation to the transducer, and the plane of the other biplane image can be varied in relation to the fixed reference image. In a preferred embodiment one image can be rotated relative to the other, and can be tilted relative to the other. An image orientation icon is shown on the display screen together with the two biplane images depicting the relative orientation of the two planar images.

RELATED APPLICATION

This is a continuation in part application of U.S. patent applicationSer. No. 09/641,306, filed Aug. 17, 2000 and now U.S. Pat. No.6,443,896.

TECHNICAL FIELD

This invention relates generally to ultrasonic imaging and, moreparticularly, to creating multiple planar ultrasonic images of avolumetric region of the body in real-time.

BACKGROUND

A major advantage of three-dimensional ultrasonic imaging is the abilityit provides to obtain unique image planes through the volume of anobject such as a human body, image planes not available throughconventional two-dimensional scanning. For example, throughthree-dimensional imaging techniques one can look simultaneously atseveral different cut planes of a region of tissue to thereby observefeatures from different angles or views. Alternatively, it may bedesirable in certain instances, to view an image plane at a constantdepth below the object surface such as the skin; such an image planecannot be obtained with normal two-dimensional scanning because of theorientation of the ultrasonic probe relative to the object.

With the ability to acquire multiple image planes of a volumetric regioncomes the need to define the planes to be imaged, their relationship toeach other in space, and the best way to display the images. In thepast, a common display technique has been to display three ultrasoundimages of a volumetric region which are of mutually orthogonal planes.Each image has two orthogonal cross-hairs displayed over it, depictingthe positions of the other two orthogonal image planes. As thecross-hairs are dragged to different positions, a new parallel imageplane in that dimension is selected and displayed. This displaytechnique enables the clinician to survey and define tissue structuresin a volumetric region by their appearances in intersecting imageplanes.

Such a display is useful for static image data of a volumetric region,which can readily be appropriately readdressed for display of differentimage planes as the selection cross-hairs are moved. The displaytechnique does not lend itself to real-time imaging, as the complexityof control and display would be increased significantly for real-timeimaging. Furthermore, such a real-time display can present too muchinformation for the clinician to analyze in a methodical or organizedmanner. Hence there is a need for effective display and control ofmultiple real-time planar images of a volumetric region.

SUMMARY OF THE INVENTION

In accordance with the principles of the present invention, method andapparatus are describe for creating and displaying multiple planarimages of a volumetric region of the body. In one aspect of theinvention, two real-time image planes are acquired and displayed in whatis referred to herein as a “biplane” display format. The two planes ofthe biplane display can be controlled in two control modes, one in whichone image plane is tilted relative to the other, and another in whichone image plane is rotated relative to the other. In another aspect ofthe invention, an icon is displayed concurrently with the biplane imagesto inform the clinician as to the relative orientation of the two imageplanes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an ultrasonic diagnostic imaging systemconstructed in accordance with the principles of the present invention.

FIGS. 2A and 2B show a display, in real time, of planar images createdby use of a two dimensional array transducer with the system of FIG. 1.

FIG. 3 illustrates in block diagram form a second embodiment of anultrasonic diagnostic imaging system constructed in accordance with theprinciples of the present invention.

FIG. 4 illustrates a biplane display when operating in the “rotate”mode.

FIGS. 5A-5D illustrate the plane orientation icon of FIG. 4 fordifferent image plane orientations.

FIG. 6 illustrates a biplane display when operating in the “tilt” mode.

FIG. 7 is a photograph of an actual ultrasound system display whenoperating in the rotate mode in accordance with the principles of thepresent invention.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of an ultrasonic diagnostic imaging system 100with which methods and apparatus in accordance with the invention can beused. It should be understood that the invention is not limited to usewith this imaging system but is shown implemented therein only as anexample. In the imaging system 100, a central controller 120 commands atransmit frequency control 117 to transmit a desired transmit frequencyband. The parameters of the transmit frequency band, f_(tr), are coupledto the transmit frequency control 117, which causes a transducer 112 ofan ultrasonic probe 110 to transmit ultrasonic waves in the selectedfrequency band. It will be understood, of course, that any ultrasonicfrequency or group of frequencies, known as a frequency signature, maybe used, with due consideration of the desired depth of penetration andthe sensitivity of the transducer and ultrasonic system.

The transducer 112 of the probe 1110 comprises an array of discreteelements that transmit ultrasonic energy in the form of a beam, andreceive echo signals returned in response to this transmission. The beamcan be steered to scan different parts of an object by mechanicallymoving the probe or, preferably, by electronically adjusting the timingof the transmission for the various array elements. In image system 100,this steering is controlled by central controller 120. The controller120, in turn, responds to commands from user entered via a userinterface 119 that includes an interface program and a pointing device(such as a mouse, trackball, stylus, tablet, touch screen or otherpointing device), keyboard, or other input device for conveyinginstructions to the central controller. Alternatively, the controllermay be programmed to steer the beam automatically in a predetermined,default manner. The received signals are coupled through atransmit/receive (T/R) switch 114 and digitized by an analog-to-digitalconverter 115. The sampling frequency f_(s) of the A/D converter iscontrolled by the central controller 120. The desired sampling ratedictated by sampling theory is at least twice the highest frequencyf_(c) of the received echoes. Sampling rates higher than the minimumrequirement can also be used. The signal samples are delayed and summedby a beam former 116 to form coherent echo signals. The coherent echosignals are then filtered by a digital filter 118 to a desired passband.The digital filter 118 can also shift the frequency band to a lower orbaseband frequency range. The characteristics of the digital filter arecontrolled by the central controller 120, which provides the filter withmultiplier weights and decimation control. Preferably the arrangement iscontrolled to operate as a finite impulse response (FIR) filter, andperforms both filtering and decimation. A wide range of filtercharacteristics is possible through programming of the weighting anddecimation rates of the filter, under control of the central controller120. The use of a digital filter allows the advantage of flexibility inproviding different filter characteristics. A digital filter can beprogrammed to pass received fundamental frequencies at one moment, andharmonic frequencies at the next. The digital filter can thus beoperated to alternately produce images or lines of fundamental andharmonic digital signals, or lines of different alternating harmonics ina time-interleaved sequence, simply by changing the filter coefficientsduring signal processing.

From the digital filter 118, the filtered echo signals are detected andprocessed by a B mode processor 137, a contrast signal detector 128, ora Doppler processor 130. The B mode processor performs functions thatinclude, but are not limited to, frequency compounding, spatialcompounding, harmonic image formation, and other typical B modefunctions that are well known in the art. The Doppler processor appliesconventional Doppler processing to the echo signals to produce velocityand power Doppler signals. The outputs of the processors 137 and 130 andcontrast signal detector 128 are coupled to a video processor 140 fordisplay as a two-dimensional ultrasonic image on the display 150. Thecentral controller 120 keeps track of the sequence of the incomingsignals, and so enables the video processor 140 to place the currentdata in the forming image. As signals are received by the videoprocessor 140, the data is fed to the display, producing rasterizedimages. The outputs of the two processors and contrast signal detectorare also coupled to a three-dimensional image rendering processor 162for the rendering of three-dimensional images, which are stored in a 3Dimage memory 164 and provided from there to the video processor 140.Three-dimensional rendering may be performed in a conventional manner.With this arrangement, an operator can select among the outputs of thecontrast signal detector 128 and the processors 137 and 130 for two- orthree-dimensional display of an ultrasonic image.

The system of FIG. 1, through the operation and control of the probe110, transducer 112, the video processor 140, and/or the image renderingprocessor 162, provides the ability to create multiple real-time planarimages of a volumetric region of an object such as a human body, whilethe body is being scanned. These planar images, when taken as slicesthrough a body, have known geometric relationships to each other,enabling a diagnostician to view body features from differentorientations. The clinician may wish to adjust the relative angles ofthe slices to visualize spatial relationships of tissue features.Through user interface 119, an operator can adjust the orientation ofthe slices displayed to align them with the features of interest in theimage. Real-time performance is achieved by generating only certainultrasonic beams needed to construct the desired planar images, ratherthan the much greater number of beams that would have to be transmittedto scan the entire volumetric region.

FIGS. 2A and 2B show an embodiment of a transducer 500 that can be usedto obtain data from a set of planes 510 and 512. This embodimentgenerates beams such as beam 504 that lies in plane 510, intersectingpoints 514 and 506; also beam 505 that lies on plane 512, intersectingpoints 516 and 508. The rays emanating from two-dimensional arraytransducer 500 can be electronically steered in three dimensions, thusavoiding the need to mechanically sweep the transducer across thevolumetric region of interest. In similar fashion, data is received fromthe lines of interest in the respective planes using well-known beamsteering and focusing and/or gating techniques applicable to atwo-dimensional array transducer.

The above scanning method for generating two planar images is preferredbecause of its speed, but is not exclusive. Variations are possible. Ifdesired, additional beams can be generated which lie in and therebydefine additional planes, or intersect additional surfaces. Eachadditional beam, of course, takes additional time to generate andtherefore affects the sweep rate. The desired number of planes and theirorientation is conveyed to central controller 120 through user interface119. In addition, the transducer 112 can be controlled to emit beamsdirected toward more than one point in each plane. Alternatively, thetransducer can be controlled to emit beams at fewer than all surfaces ateach sampling position, as long as the beams lie in at least two planes,or intersect at least two non-planar surfaces, or lie in at least oneplane and intersect at least one non-planar surface, per sweep. Theseand other obvious variations can produce multiple planar images inreal-time, but at different rates and with different resolutions,depending on the variation chosen. Furthermore, any two-dimensionalultrasonic imaging technique, for example, B mode, contrast signaldetection, harmonic imaging, or Doppler imaging, can be applied equallywell with this data acquisition scheme.

The data acquired from the two planes 510 and 512 are used by one ormore of the processors 137, 130, or the contrast signal detector 128 toconstruct the corresponding planar images. The planar images arepreferably created at a scanning rate to provide real-time imaging. Theplanar images can be simultaneously displayed side-by-side by the videoprocessor 140, or in a three dimensional perspective view on the display150 as the volumetric region is continuously scanned, or viewed later.

FIG. 3 illustrates another embodiment of an ultrasound systemconstructed in accordance with the principles of the present invention.In this embodiment the probe 110 includes a two dimensional arraytransducer 500 and a micro-beamformer 502. The micro-beamformer containscircuitry which control the signals applied to groups of elements(“patches”) of the array transducer 500 and does some processing of theecho signals received by elements of each group. Micro-beamforming inthe probe advantageously reduces the number of conductors in the cable503 between the probe and the ultrasound system and is described in U.S.Pat. No. 5,997,479 (Savord et al.) and in U.S. Pat. No. 6,436,048(Pesque).

The probe is coupled to the scanner 310 of the ultrasound system. Thescanner includes a beamform controller 312 which is responsive to a usercontrol and provides control signals to the microbeamformer 502instructing the probe as to the timing, frequency, direction andfocusing of transmit beams. The beamform controller also control thebeamforming of received echo signals by its coupling to theanalog-to-digital (A/D) converters 316 and the beamformer 116. Echosignals received by the probe are amplified by preamplifier and TGC(time gain control) circuitry 314 in the scanner, then digitized by theA/D converters 316. The digitized echo signals are then formed intobeams by a beamformer 116. The echo signals are then processed by animage processor 318 which performs digital filtering, B mode detection,and Doppler processing as described above, and can also perform othersignal processing such as harmonic separation, speckle reduction throughfrequency compounding, and other desired image processing.

The echo signals produced by the scanner 310 are coupled to the digitaldisplay subsystem 320, which processes the echo signals for display inthe desired image format. The echo signals are processed by an imageline processor 322, which is capable of sampling the echo signals,splicing segments of beams into complete line signals, and averagingline signals for signal-to-noise improvement or flow persistence. Theimage lines are scan converted into the desired image format by a scanconverter 324 which performs R-theta conversion as is known in the art.The image is then stored in an image memory 328 from which it can bedisplayed on the display 150. The image in memory is also overlayed withgraphics to be displayed with the image, which are generated by agraphics generator 330 which is responsive to a user control. Individualimages or image sequences can be stored in a cine memory 326 duringcapture of image loops.

For real-time volumetric imaging the display subsystem 320 also includesthe 3D image rendering processor 162 which receives image lines from theimage line processor 322 for the rendering of a real-time threedimensional image which is displayed on the display 150.

In accordance with the principles of the present invention, two images,referred to herein as biplane images, are acquired by the probe in realtime and displayed in a side by side display format. Since the 2D array500 has the ability to steer transmitted and received beams in anydirection and at any inclination in front of the array, the planes ofthe biplane image can have any orientation with respect to the array andto each other, as shown by the orientation of image planes 510, 512 tothe array 500 in FIGS. 2A and 2B. However in a preferred embodiment thetwo image planes intersect the center of the array 500 and areorthogonal to the sides of the array as shown by planes L and R in FIG.5B, in which the planes are viewed “edge-on” from the array transducer.In the examples given below the image format is the sector image format,with the image lines emanating from a near-field apex. However, linearor steered linear scan formats can also be employed.

The biplane images in the two image planes are acquired by transmittingand receiving beams of each image as exemplified by the acquisition ofbeams 504 and 505 in the respective image planes of FIG. 2A. Variousacquisition sequences can be performed. All of the scanlines of oneimage can be acquired, followed by acquisition of all of the scanlinesof the other image. Alternatively, acquisition of the lines of the twoimages can be time interleaved. For instance, line 1 of one image can beacquired, followed by the acquisition of line 1 of the other image. Thiswould be followed by the acquisition of line 2 of each image, then line3 of each image, and so forth. This may be advantageous when doingDoppler image of low flow velocities, as the interval betweeninterrogations of an ensemble of lines can be lengthened. It alsoadvantageously results in the lines at the intersection of the twoplanes being acquired in succession, which prevents rapidly movingtissue at the image intersection from appearing different in the twoimages. The lines can be acquired in their spatial progression in theimage, or sequentially from separated portions of the image. Forinstance, the four edge lines can be acquired first, followed by thefour central lines around the intersection of the planes, thenprogressing alternately toward and away from the intersection.

When all of the lines of both images have been received by the scanner310 and forwarded to the display subsystem 320, the scanner sends an“EK” signal over control lines 340 to the display subsystem, telling thedisplay subsystem that all of the lines for the current display framehave been sent for display. The display subsystem then processes theimage lines for display. For the biplane format described below, oneimage is processed, formatted and mapped for display on one side of thedisplay screen and the other image is processed, formatted and mappedfor display on the other side of the display screen. After the imageshave been processed the display subsystem returns an “FRQ” controlsignal to the scanner, informing the scanner that the display subsystemis requesting another image frame for processing. The complete screendisplay of two side-by-side images is overlaid with the graphics for theimages and displayed on the display 150. The display subsystem thenawaits the completion of another scanning of the two images as indicatedby the concluding receipt of another EK signal, at which time theprocessing and display of another real time display frame proceedsagain.

It is also possible to use a communication architecture in which eachimage is concluded with an EK signal and the transmission and receipt ofboth biplane images, each concluded by an EK signal and responded to byan FRQ signal, is done before a two-image display frame is produced bythe display subsystem.

The images are displayed side-by-side as illustrated graphically byimages L and R in FIG. 4 and by the photograph of the system displayshown in FIG. 7. In a preferred embodiment the image plane orientationsare selected by one of two selection modes, “rotate” or “tilt.” In apreferred embodiment, the orientation of one image, the left image L inFIG. 4, is fixed in relation to the transducer array. The L image isalways in a plane which is orthogonal to the plane of the array,extending through the center of the array as shown in FIG. 2B. The planeof the right image R can be rotated or tilted by user control relativeto the plane of image L. In the rotate mode, the two images always sharea common center line during sector imaging, and the plane of the rightimage R can be rotated by manipulation of a user control such as atrackball or knob. The right image can be rotated from being co-planarwith the left reference image to a 90° orientation and through toco-planar again. A full 360° of rotation is possible either bymanipulation of the user control or by left-to-right inversion of theimage. In the tilt mode the center of the right image R alwaysintersects the reference image, but can be tilted to intersect differentlines of the reference image as if the sector is swinging from thecommon apex of the two images.

In a preferred embodiment the probe 110 has a marker on it whichidentifies a given side of the image. Generally this marker is aphysical protrusion or color on one side of the probe case. Cliniciansuse this marker to relate the orientation of the probe to theorientation of the image on the display. It is customary to display themarker on the display screen as shown by dot 402 in FIG. 4. Theclinician will generally always hold the probe with the probe marker inthe same position so that the image always is shown with an orientationwhich the clinician prefers. In accordance with a further aspect of thepresent invention, the second image R is also shown with an orientationmarker 404. In the rotate mode the two images can both be imaging thesame plane when scanning is initiated, in which case the markers arespatially aligned. The clinician can then rotate the right image planefrom the common starting orientation. In a constructed embodiment theinitial condition of the two biplane images is that the two are aligneduntilted along a common center line and rotated 90° with respect to eachas shown in FIG. 7.

In accordance with a further aspect of the present invention, an icon400 is displayed on the biplane display to graphically indicate therelative orientation of the two image planes. The icon 400 in FIG. 4represents a view of the image planes from the transducer array and hasa circle 410 which graphically represents the space in which the base ofthe sector R can rotate. The dot 406 corresponds to the dot 402 of theleft reference image L and indicates that the plane of the referenceimage is in a horizontal orientation across the circle 410 with themarker at the right of the image. The line 412 of the icon indicatesthat the right image R is in the same orientation with the right imagemarker 408 (corresponding to dot 404) at the right side of the image.

FIGS. 5A-5D illustrate how the icon 400 changes as the right image isrotated. When the right image is rotated 30° from the plane of thereference image, the icon 400 will appear as shown in FIG. 5a, in whichthe line 412 and dot 408 representing the plane of the right image haverotated thirty degrees. The number 30 also appears below the icon. Theright image plane can be rotated another 180°, in which case the line412 and marker dot 408 will appear as shown in FIG. 5B. The number belowthe icon changes to 210 to indicate a 210 degree orientation to thereference image plane. Alternatively, in the preferred embodiment theuser interface of the ultrasound system includes a “right image invert”control. When this control is actuated, the right image will immediatelyinvert laterally by 180°, and the icon will correspondingly switch fromthat shown in FIG. 5A to that shown in FIG. 5B.

Similarly, the preferred embodiment includes a “left image invert”control which laterally inverts the left image. FIG. 5C illustrates theicon when the reference image has been inverted, in which case themarker dot 406 is at the left side of the icon. In FIG. 5C the rightimage is at a 210 degree orientation to the original (uninverted)position of the reference image as shown by line 412 and the numberbelow the image. In FIG. 5D the reference image has been inverted withthe right image at a 30° orientation to the original position of theleft reference image.

An advantage of the common display of the biplane images and the icon isthat when the images on the display screen are saved, the icon is alsosaved without further effort by the sonographer. During later review ofthe images by a clinician the orientation of the two image planes isshown on the display or in the print of the screen. The screen displaycan be saved either in hard copy or electronically, and can be retrievedand referred to later to enable the patient to be scanned again with thesame biplane image orientation.

It may be desirable to have the icon 400 graphically indicate theportion of the rotational circle 410 which corresponds to 0°-180°, andthe portion which corresponds to 181°-359° in the numeric notationdisplayed below the icon. This may be done by using visiblydistinguishable graphics for the lower and upper halves of the circle410. For instance the lower half of the circle 410 could be displayedwith a brighter or bolder line than the upper half, or could be dottedor dashed while the upper half is drawn with a solid line.Alternatively, the lower and upper halves could be differently colored,blue and green for instance, with the color of the numeric notationchanged correspondingly with changes in the rotational angle of theright plane R.

FIG. 6 illustrates the display screen when operating in the “tilt” mode.In this mode the plane of the left image L is again fixed relative tothe plane of the transducer array, and the right image R can be tiltedfrom one side of the reference image to the other as if swinging fromthe common apex of the two images. In a constructed embodiment the twoplanes are always oriented 90° to each other in the lateral (rotational)spatial dimension. In a preferred embodiment the center line of theright sector image R always intersects the reference image, but at aline of the left sector which is selected by the user. An icon 600indicates the relative orientation of the two image planes. In the icon600 the small graphical sector 602 represents the fixed position of theleft reference image. A cursor line 604 represents the right imageviewed “edge-on” from the side. In this example the right image plane istilted 30° from a nominal orientation in which the center lines of thetwo images are aligned, which is a 0° reference orientation. In thenominal (initial) orientation the cursor line is vertically oriented inthe icon 600.

As an alternative to the icon 600, the cursor line 604 can be displayedover the reference image L. The user can manipulate a user control tochange the tilt of the right plane R, or can drag the cursor line fromone side of the image R to the other to change the tilt of the rightplane. Cursor display types other than a line, such as dots or pointers,can also be used for cursor 604.

The tilt mode is particularly useful for conducting longitudinal studiesof infarcts. Suppose that cardiac imaging of a patient reveals abnormalheart wall motion in the vicinity of the papillary muscle tips. Withconventional 2D imaging, the clinician may try to image the infarctedwall by first acquiring an image of the papillary muscle in a long axisview of the heart, then rotating the probe ninety degrees to image theinfarct location in a short axis view. However, if the probe (and hencethe image plane) is not precisely rotated, the clinician can miss theinfarct location. With the biplane tilt mode, the clinician can move theprobe until the papillary muscle is shown in the reference image in along axis view, then can tilt the cursor line 604 to point to or overlaythe papillary muscle tips in the long axis reference image, therebybringing the infarcted location into view in the tilted right image R ina short axis view. When the clinician wants to view the same section ofthe heart wall in a short axis view three or six months later in alongitudinal study, the process of imaging the papillary muscle in along axis view in the left image, pointing the tilt cursor 604 in thesame inclination, and viewing the infarcted region in a short axis viewin the right image can be precisely repeated, thereby improving thediagnostic efficacy of the longitudinal study.

FIG. 7 shows two biplane images in the rotate mode. The icon between thetwo images in the center of the screen shows that the right image planehas been rotated ninety degrees from alignment with the left referenceimage plane. The marker dots are clearly visible in the icon and on theright sides of the apexes of the two sector images. For completeness ofa cardiac study the EKG trace is also shown below the biplane images.

An advantage of the present invention is that since only two planes of avolumetric region are being imaged, acquisition of the two images can bedone rapidly enough so that the two images can both be real-timeultrasonic images at a relatively high frame rate of display. A furtheradvantage is that the ultrasound system need be only a conventional twodimensional imaging system. As FIG. 3 shows, the display subsystem forbiplane imaging can be a conventional two dimensional image processingsubsystem, which means that biplane imaging in accordance with thepresent invention can be done with the two dimensional ultrasoundsystems currently in the hands of clinicians. The scanner and displaysubsystem of FIG. 3 needs no unique 3D capabilities in order to producethe biplane image shown in FIG. 7.

The tilt and rotate modes can be combined, enabling a user to viewbiplane images which are both tilted and rotated relative to each other.

What is claimed is:
 1. An ultrasonic diagnostic imaging systemcomprising: a two dimensional array transducer; a beamformer coupled tothe array transducer which beamforms received echo signals; acontroller, coupled to the array transducer, which controls thetransducer to scan two image planes of a volumetric region whilescanning less than the entire volumetric region, the two image planesexhibiting a spatial orientation within the volumetric region; a displaysubsystem, coupled to the beamformer, which produces real-time images ofthe two image planes shown concurrently on a common display; and a usercontrol, responsive to actuation by a user and coupled to thecontroller, which selects the orientation of the plane of one of the twoimages relative to the plane of the other image.
 2. An ultrasonicdiagnostic imaging system comprising: a two dimensional arraytransducer; a beamformer coupled to the array transducer which beamformsreceived echo signals; a controller, coupled to the array transducer,which controls the transducer to scan two image planes of a volumetricregion, the two image planes exhibiting a spatial orientation within thevolumetric region; a display subsystem, coupled to the beamformer, whichproduces real-time images of the two image planes shown concurrently ona common display; and a user control, responsive to actuation by a userand coupled to the controller, which selects the orientation of theplane of one of the two images relative to the plane of the other image,wherein the controller comprises a scan plane controller which controlsthe transducer to scan two intersecting scan planes, and wherein theuser control selects the rotational orientation of one of the scanplanes relative to the other.
 3. The ultrasonic diagnostic imagingsystem of claim 2, wherein one of the scan planes exhibits a fixedrotational orientation relative to the plane of the transducer array,and wherein the rotational orientation of the other scan plane isresponsive to the user control.
 4. The ultrasonic diagnostic imagingsystem of claim 3, wherein both scan planes are orthogonal to the planeof the transducer array.
 5. An ultrasonic diagnostic imaging systemcomprising: a two dimensional array transducer; a beamformer coupled tothe array transducer which beamforms received echo signals; acontroller, coupled to the array transducer, which controls thetransducer to scan two image planes of a volumetric region, the twoimage planes exhibiting a spatial orientation within the volumetricregion; a display subsystem, coupled to the beamformer, which producesreal-time images of the two image planes shown concurrently on a commondisplay; and a user control, responsive to actuation by a user andcoupled to the controller, which selects the orientation of the plane ofone of the two images relative to the plane of the other image, whereinthe controller comprises a scan plane controller which controls thetransducer to scan two intersecting scan planes, and wherein the usercontrol selects the angular orientation of one of the scan planesrelative to the other.
 6. The ultrasonic diagnostic imaging system ofclaim 5, wherein one of the scan planes exhibits a fixed angularorientation relative to the plane of the transducer array, and whereinthe angular orientation of the other scan plane is responsive to theuser control.
 7. The ultrasonic diagnostic imaging system of claim 6,wherein the scan plane exhibiting a fixed angular orientation relativeto the plane of the transducer array is orthogonal to the plane of thetransducer array.
 8. An ultrasonic diagnostic imaging system comprising:a two dimensional array transducer operated to scan first and secondimage planes of a volumetric region at a real time scanning rate whilescanning less than the entire volumetric region; a beamformer coupled tothe array transducer which beamforms received echo signals; a twodimensional display subsystem, coupled to the beamformer, which isresponsive to the received echo signals to produce two dimensional imagedisplay frames at a real time display rate, each image display frameincluding images of the first and second image planes; and a displaywhich displays the two dimensional image display frames as real timeimages.
 9. The ultrasonic diagnostic imaging system of claim 8, whereinthe two dimensional array transducer is operated to first scan the firstimage plane to acquire the scanlines of a first complete image, and thento scan the second image plane to acquire the scanlines of a secondcomplete image.
 10. The ultrasonic diagnostic imaging system of claim 9,wherein the display subsystem first receives all of the echo signals ofan image from the first image plane which are used to process a portionof an image display frame; and wherein the display subsystem receivesall of the echo signals of an image from the second image plane whichare used to process the remaining portion of the image display frame.11. An ultrasonic diagnostic imaging system comprising: a twodimensional array transducer operated to scan first and second imageplanes of a volumetric region at a real time scanning rate; a beamformercoupled to the array transducer which beamforms received echo signals; atwo dimensional display subsystem, coupled to the beamformer, which isresponsive to the received echo signals to produce two dimensional imagedisplay frames at a real time display rate, each image display frameincluding images of the first and second image planes; and a displaywhich displays the two dimensional image display frames as real timeimages, wherein the two dimensional array transducer is operated toalternately acquire less than all of the scanlines of a complete imagefrom the first image plane and less than all of the scanlines of acomplete image from the second image plane in a time interleaved manneruntil a complete image has been acquired from both image planes.
 12. Theultrasonic diagnostic imaging system of claim 8, further comprising auser control, coupled to the two dimensional array transducer, whichselects the relative spatial orientation of the first and second imageplanes to each other.
 13. A method of producing biplane ultrasonicimages comprising: scanning two spatially oriented image planes of avolumetric region in real time with a two dimensional array transducerwhile scanning less than the entire volumetric region; producing realtime images in which images of the two image planes are concurrentlydisplayed in real time; changing the relative spatial orientation of theimage planes being scanned; and producing real time images in whichimages of the two image planes in their new relative spatial orientationare concurrently displayed in real time.
 14. A method of producingbiplane ultrasonic images comprising: scanning two spatially orientedimage planes of a volumetric region in real time with a two dimensionalarray transducer; producing real time images in which images of the twoimage planes are concurrently displayed in real time; changing therelative spatial orientation of the image planes being scanned; andproducing real time images in which images of the two image planes intheir new relative spatial orientation are concurrently displayed inreal time, wherein changing the relative spatial orientation of theimage planes further comprises maintaining the spatial orientation ofone of the image planes relative to the plane of the array transducer.15. The method of claim 13, wherein scanning further comprises scanningtwo image planes which intersect at a line; and wherein changingcomprises changing the rotational orientation of at least one of theimage planes about the line.
 16. The method of claim 13, whereinscanning further comprises scanning two image planes which intersect ata line; and wherein changing comprises changing the location of the linerelative to at least one of the image planes.
 17. An ultrasonicdiagnostic imaging system comprising: a two dimensional arraytransducer; a beamformer coupled to the array transducer which beamformsreceived echo signals; a controller, coupled to the array transducer,which controls the transducer to scan two image planes of a volumetricregion defined by the possible locations of the two image planesrelative to the array transducer while scanning less than the entirevolumetric region, the two image planes exhibiting a spatial orientationwithin the volumetric region; a display subsystem, coupled to thebeamformer, which produces real-time images of the two image planesshown concurrently on a common display; and a user control, responsiveto actuation by a user and coupled to the controller, which selects theorientation of the plane of one of the two images relative to the planeof the other image.
 18. A method of producing biplane ultrasonic imagescomprising: scanning two spatially oriented image planes of a volumetricregion defined by the possible locations of the two image planesrelative to an array transducer in real time with a two dimensionalarray transducer while scanning less than the entire volumetric region;producing real time images in which images of the two image planes areconcurrently displayed in real time; changing the relative spatialorientation of the image planes being scanned; and producing real timeimages in which images of the two image planes in their new relativespatial orientation are concurrently displayed in real time.